This invention relates to a flat type fluorescent display tube and in particular to a fluorescent display tube for use in a desk-top type electronic calculator.
Generally, a fluorescent display tube is driven at a low voltage of scores of volts and it is frequently used as a display for a desk-top type electronic calculator, since it has a sufficient brightness from the standpoint of visibility. A cylindrical type display tube has been conventionally used, but it is being recently replaced by a flattened box type multi-digit display tube in view of a small size requirement, as well as a flatness requirement, of the desk-top electronic calculator. A flat type display tube has been proposed in which lead-in conductors such as current supply lead-in strips and exhaust tube extend in the same direction from a body of the display tube. In such a type, the lead-in strips and exhaust tube are disposed close to each other, offering the following disadvantages. That is, no efficient sealing operation can be effected on the exhaust tube due to the close proximity of the lead-in strips. It is preferred that such a sealing operation be automated, but the neighboring lead-in strips provide a bar to such an automatic operation. Furthermore, the lead-in strips are tarnished, during the air evacuation of the tube, due to oxidation and a further treatment such as polishing etc. is required in removing the oxidation film from the lead-in strip. Since in this case the lead-in conductor is in close proximity to the exhaust tube, there is a danger that the exhaust tube will be broken when polishing the tarnished lead-in conductor. A soldering operation is effected in connecting the lead-in conductor to an associated external circuit. At this time there is also a danger that the exhaust tube will be broken due to a heat being transmitted during the soldering operation from the lead-in conductor to the exhaust tube. With these disadvantages in view a modified display tube is proposed at the expense of a small size requirement, in which an exhaust tube extends in one direction and lead-in strips extend in a direction opposite to that of the exhaust tube. However, there is a growing demand for a flat type desk-top calculator of small size and dimension. In view of a small size requirement the calculator is so unreasonably designed that the exhaust tube of the display tube is in direct contact with the casing of the calculator. In this case, a crack will be developed in the exhaust tube of the built-in display tube due to vibration etc. applied on the calculator from the outside. The crack in the exhaust tube breaks a vacuum in the display tube, failing to cause the display tube to be driven.